The aforesaid principle of detecting pressure by permitting the pressure to act upon an optic fibre made of glass or a plastics material, so that the transmitted light is periodically disturbed, is described in, for example, U.S. Pat. No. 4,163,397; SE-A-No. 410 521 and EP-A-No. 008 2820. According to EP-A-No. 008 2820 this periodic disturbance is created by winding a filament or wire helically around the optical fibre, whereafter an outer sheathing is placed around the filament and fibre structure. When light is transmitted through the fibre and the sheathing is simultaneously subjected to a pressure force, this pressure force will tend to flatten the wire helix. In this way there is created a series of periodic bends in the fibre, each of which corresponds to half the pitch of the helix. As a result thereof, the light passing through the fibre is attenuated, which can be indicated, for example, with the aid of a photoelectric sensor. Various fibre parameters affect the attenuation of the light. In the case of a fibre which exhibits a parabolic index profile, the periodic disturbance has a critical period length which produces maximum attenuation. This period length can be expressed as ##EQU1##